1. Field of the Invention
The present invention relates to a coal or mineral washing jig for separating the useful coal or mineral product from refuse, such as rock. More specifically, the invention relates to a nuclear density based jig control system for use in such washing jigs.
2. Background of the Invention
Coal washing jigs have long been utilized to separate useful coal from rock and other refuse in mined material. Jigging is a process which hydraulically separates and stratifies particles to be cleaned according to their density. The process is created through a complex pulsating water cycle that lifts a bed of material to be cleaned so that the particles settle according to their specific gravity; while at the same time moving the material laterally or longitudinally to a weir so that the top stratum is useful coal and can be moved along for further cleaning. In this manner, the coal is separated from the bottom refuse layer of rock which is removed from the cycle.
The basic construction in a well-known "Baum"-type jig comprises a series of components arranged end to end which provide multiple washing compartments allowing for primary and secondary separations. Jigs are generally constructed of two or more compartments wherein each compartment is fitted with a perforated screen plate and a hydraulically operated refuse ejector. The compartments are subdivided into cells which are placed end to end such that the washed product from one feeds the next.
The material from the mine to be cleaned containing useful coal and refuse enters the jig and establishes a bed upon the perforated screen plate. This bed comprising solids and water is alternately pulsed up and down by the jigging action affecting the separation of heavier refuse and lighter coal. The bed is simultaneously moved longitudinally in the jig concurrently with the pulsing action.
The successful operation of a coal washing jig is directly related to the refuse removal control system. A bed depth sensing mechanism is utilized in the Baum-type jig to determine when the amount of rock and heavy contaminants in the jig have reached a point at which they must be discharged to prevent contamination of the washed coal product. This device, called a "float mechanism", comprises a triangular aluminum float which rides in the jig bed at a density level that is adjustable via the specific gravity control weight in the float arm.
The refuse removal system consists of an air cylinder, refuse gate and linkage connecting the two. Its function is to control the removal of the refuse from the bottom of the jig bed. Enough of the bottom strata of the jig bed must be withdrawn to prevent contaminants from entering the upper coal layer yet excessive removal must be controlled since valuable coal should not be withdrawn through the gate. When the refuse gate is closed, rock and heavy impurities are prevented from exiting the jig bed. Coal product, however, continuously overflows the weir above the refuse gate and passes out of the jig As a new feed enters the jig, the rock and impurities build up, causing the aluminum float to gradually rise. As the float rises, the rocker valve on which it pivots will tilt until air is channeled to the air cylinder. The cylinder will rise and, through the connecting linkage, cause the refuse gate to be opened. The bottom layer of refuse will then pass through the gate and the float will lower, causing the rocker valve to tilt back. The air supply to the cylinder will be cut off and, as the float continues to drop, the valve will exhaust the air in the cylinder to the atmosphere, causing the refuse gate to lower, thus halting the removal of the refuse. The refuse now begins to accumulate again and the cycle repeats itself.
There are many disadvantages of the float actuated gate system. The float is slow to respond to changes in the bed density, and requires visual observation by an operator to verify correct operation. The system is susceptible to malfunctioning due to undetected float wear such that the float position no longer accurately reflects the actual position of the top layer of the refuse layer. The float system exhibits a frequent jamming of the float within the bed. Furthermore, pivot bearing wear of the float mechanism affects the performance of the device, as do changes in the pulsation intensity of the jigging apparatus.
U.S. Pat. No. 3,082,873 to Bartelt discloses a replacement for float actuated systems wherein the measurement of the bed density in a jig is determined through electromagnetic radiation. Bartelt utilizes radiation sources 7 which are mounted outside the bed 2 on the side of the jig. One or two detectors 5 and 5' are adjustably mounted on a cross bear 6 and are located within the bed 2. The detectors 5 and 5' are elongated in the direction of the horizontal flow of the bed. The point source 7 and the elongated detector 5 define a horizontal zone of measurement in which the density of the bed of material is determined. The zone of measurement is triangular in plan and of small vertical thickness. The apparatus for classifying particulate material disclosed in Bartelt utilizes the detectors to detect the radiation from the source which passes through the material lying within the zone of measurement. The amount of radiation detected by the detector will vary inversely with the specific density of material within the zone of measurement. A device uses this determination of the specific gravity of material within the zone of measurement to operate the refuse gate. Bartelt also discloses the use of two radiation detectors positioned offset one from the other. In this case, the electrical signals from both the radiation detectors are led to the rapid acting control device for the refuse gate. The control signal is derived from the difference between the measurements given by the two radiation detectors.
The system disclosed in Bartelt suffers from several disadvantages. In Bartelt's preferred arrangement the radiation sources 7 are placed outside of the bed. This results in greater exposure of radiation to the surrounding area including workers in the immediate area. Additionally, Bartelt fails to disclose any shielding mechanism to minimize and contain the radiation sources when not in use. Bartelt discloses that the sources may be provided within the bed; however, the placement of the detectors 5 in the middle of the bed as taught by Bartelt requires that the sources 7, even if within the bed, be positioned near the edges of the jig bed and still results in exposure of the surrounding environment to radiation.
The desired zone of measurement disclosed in Bartelt is specifically set up to be of minimal vertical thickness. This minimal thickness of the measurement zone results in a detection of the specific gravity of only a small band of the bed of material Testing of such a minimal slice increases the potential for misrepresentation of the overall specific density of the entire lower layer due to fluctuations in the bed such as due to large chunks of refuse material or the like.
Bartelt also discloses the use of preferably a point like source of radiation or one which extends in the horizontal direction of the bed. Thus, Bartelt is unable to measure bed density over a vertical height.
Bartelt also only provides a single location for the positioning of detectors basing the reliability of the results on the accuracies of individual detectors.